If you are having problems downloading the latest Windows 10 Mobile Redstone 2 build you are not alone. Many devices (including my Lumia 950 XL) are sticking on 0% after detecting the update. Microsoft have acknowledged the problem with a thread on the Microsoft Answers forum.
The problem with build 14951 seems to be related to battery levels with Microsoft recommending having devices plugged in during the upgrade. So far my Lumia is still not downloading the update despite being on charge, I will see if it still happens when the device is 100% charged and I restart it.
Have a look at the forum thread for more details.
Hello Windows Insiders!
We’ve just launched build 14951 to the Fast ring today and we’re seeing reports from users staying their device is not progressing past “0% downloading” on the update screen.
We encourage Windows Insiders to file feedback or upvote feedback from others. We’ve started digging through the logs we have already received to investigate the issue.
Stay tuned to this thread for additional details. We’ll post updates as information becomes available.
Thanks in advance!
*** Edit 2016.10.19 – 9:45PM ***
In reviewing device logs from feedback submissions, some devices are needing to be plugged in. As an interim step, please plug your device into a charger and allow it to fully charge if it is not.